CADR calculator: What size air purifier do I need?

The honest truth is that in the world of clean air, size does matter
Updated on April 3, 2024
Written by
Teddy Booth
Teddy joined HouseFresh in 2019 as a senior content producer, producing written content as well as product photography and YouTube videos. He supports Danny as a product tester and is our lead how-to writer.

Different models of air purifiers are specifically designed for rooms of different sizes. Choosing the wrong size will be a waste of money and energy and may not have the power to clean the pollutants from your air effectively.

By taking the time to measure your room and having an understanding of CADR and air changes per hour, you can find the ideal air purifier for your needs.

Science has proven that running an air purifier can drastically improve the quality of air in your home, especially as we spend around 90% of our time indoors. Units nowadays can perform various functions like giving you up-to-date air quality information, running automatically and even charging your phone wirelessly. 

But, if you want the best out of your air purifier, you must go back to basics and think about performance.

Not all air purifiers are created equal, so when you’re looking at buying one, it’s not only the budget, size or smart functions you need to consider. For an air purifier to be effective, you need a unit with a capable performance for the space you’re using it in. This is often measured using CADR.

The CADR (Clean Air Delivery Rate) of each air purifier shows us how well it can perform in a space. 

In this guide, I’m going to explain what you need to know about CADR, help you understand what we mean by “air changes per hour,” and show you how to calculate what size air purifier you will need for your space. Plus, I’ll give you a handy tool to simplify the process.

When I talk about the size of an air purifier, I’m not specifically talking about the unit’s physical size. I’m also referring to the unit performance. 

Performance often comes hand-in-hand with physical size, but not always.

Calculating the ideal size of an air purifier

One of the most important actions to take when considering an air purifier is to find a unit that will perform efficiently and effectively in your particular room. If you purchase a unit that’s too small for your space, it won’t effectively clean your air. Buying a unit that’s too big will be a waste of money, energy and floor space.

I’ll take you through how to properly measure your room, how to calculate the CADR you’ll need, how to pick the right unit for your space and where to place your air purifier to get maximum cleaning performance.

1. How to measure the size of the room

First, you need to measure your room accurately to know how much air your purifier needs to clean.

Whether you’re using a laser measure or good old-fashioned tape, you will need measurements for the length, width and height of your room:

With these three figures, you can work out your space’s square footage and volume.

2. How to calculate the CADR needed to clean the room

Brands will tell you what size room their products are intended for in square footage, but this isn’t the only information to go by. That’s why we should also work out what CADR an air purifier needs to clean your room.

The next step is calculating how much power (CADR) an air purifier will need to clean your room effectively.

I’ll admit I’m awful at math; that’s why our IT Engineer Vadym has created the HouseFresh CADR Calculator to do all the hard work so you don’t have to.

Simply fill in the dimensions of your room and the calculator will automatically work out what CADR is required to clean your space.

3. How to pick the right air purifier for the room

Manufacturers are usually forthcoming about their specs and features and use them as a selling point, so it’s not too hard to find that information. But take this data with a pinch of salt, as often their tests are carried out in sterile environments, not real-world situations.

This is why here at HouseFresh, we test every air purifier we review in our home lab so you know how it performs in a real room, in a real home.

Now that you know which specifications you need from an air purifier, how do you pick the right one for your room? 

This is where a little research goes a long way.

  1. Check the square footage of the air purifier matches the size of your room
  1. Check the CADR performance matches the data given by our CADR Calculator
  1. Double-check all these figures against our hands-on product reviews

The unit with the highest CADR is the best for my room, right?


If you have a larger room or live in a highly polluted area, a large air purifier with a high CADR is the way to go. But, if you have a small room with limited floor space, a large air purifier will be overkill.

It’s recommended that you run an air purifier 24/7, so running a high-powered unit in a smaller room will be a big waste of energy, not to mention the initial cost of the air purifier (higher CADR usually means a higher price tag). 

The ideal unit for your home should match its CADR to the size of the room you’re using it in; hence, we advise measuring your space and using our CADR Calculator.

  • Air purifiers can come with different types of filters to remove pollutants from your air. I recommend choosing an air purifier with a high-grade HEPA filter and an activated carbon filter packed with charcoal pellets.
  • Advancements in technology mean that air purifiers can come packed with smart features like air quality sensors and indicators, auto mode and app connectivity. Consider which features will benefit your needs before you make a purchase.
  • Air purifiers have a fan and a motor therefore, they make noise when running. Some are noisier than others, so be sure to check their sound level at the highest and lowest fan speeds.
  • Far gone are the days of the white box air purifiers. Now, they come in all shapes, designs and colors, so pick one that you like and/or a unit that fits with the decor of your room.

Understanding clean air delivery rate

Okay, let’s get into it. What is CADR, and how does it affect how an air purifier clears pollutants from your room?

CADR stands for Clean Air Delivery Rate and is commonly used to measure the capabilities of an air purifier by measuring its ability to clear a volume of air. CADR data is shown in CFM or Cubic Feet per Minute, so it’s important to measure your space accurately before deciding on a unit.

This system of measuring air performance was developed by The Association of Home Appliance Manufacturers (AHAM), which tests each product under specific conditions in a specially designed room. This means every air purifier is tested on an even par.

After a product is tested, the results are issued with a CADR certificate by AHAM to show its performance data.

So that’s the what; let’s look at the why because knowing how well an air purifier performs can mean the difference between breathing clean, pollutant-free air and just having an air purifier as an ornament in your home.

Buying an air purifier with a high CADR comes with many benefits;

  • They can clear pollutants from larger spaces
  • Are more efficient at removing contaminants like bacteria, allergens, dust and viruses
  • They are still highly effective at lower fan speeds

If you want to delve deeper into CADR, check out our full guide: What is CADR rating?

Understanding air changes per hour

Another acronym you’re likely to hear when looking at purchasing an air purifier is ACH, which stands for Air Changes per Hour.

ACH refers to how many times the air is replaced in a room during one hour of a unit running. If you see 4 ACH, for example, it means the air will be fully filtered and replaced with clean air four times an hour.

But Teddy, how do I know how many air changes I need? I hear you ask. This is where you need to consider how you’re using your space. 

For a room where you spend a lot of time or are creating more contaminants, let’s say a kitchen or a living room, you will want to change the air regularly as pollutants will build quickly. For a room that doesn’t have much going on, a small home office, for example, you can choose a unit with less ACH as there’s not much activity going on (pollution-wise).

You should also consider how much ventilation you have in your space. Rooms with plenty of ventilation will require less ACH and rooms with little to no ventilation will require more.

If this is all a little mind-boggling, don’t worry. We’ve created another handy tool. Introducing our ACH Calculator.

Simply fill in your room measurements and the CADR of the unit you’re looking to purchase, and our tool will calculate the ACH of that particular air purifier.

Common questions about sizing an air purifier

Yes, it can. Both in size and power.

If you have limited space in your room, the last thing you want to do is waste it on a big, unnecessary air purifier. But, as well as using up much-needed floor space, an air purifier can be too powerful for a room. 

Having a large unit in a small room is a giant waste of energy and you’ll be subjecting yourself to high running costs when a smaller, less powerful unit will do the ideal job.

You need to make certain considerations when deciding both which and how many units you need for your space.

  • Will one air purifier clean your whole room
  • How severely are you affected by pollutants
  • Will you want to move units between rooms
  • Do you suffer from allergies

If you measure your space accurately and can find an air purifier that will effectively clear your room, I would recommend getting one unit per room.

Air purifiers are designed to clean the space they are in, not your whole home. 

If your house is prone to pollution e.g. you live next to a busy road or are affected by wildfire smoke, or suffer from allergies like pet dander, dust mites or mold spores, it’s best practice to have an air purifier for each room so you are fully protected.

Closing thoughts

Not all air purifiers are created equal. There are units on the market to cater to a vast range of different room sizes, so it’s paramount you get the right unit for your particular space.

Save yourself any guesswork or disappointment by conducting the necessary measurements and using our HouseFresh Calculators; you can find the ideal air purifier to fit your needs.

Once you’ve done your measurements, check out our list of hands-on air purifier reviews, where we test units in real-world environments to give you our honest opinions and accurate performance data.


Join our newsletter

We’ll send you a nice email every once in a while. No spam.

Support HouseFresh

Buy us a coffee
Your support makes it possible for us to keep doing what de wo.
About the author

Teddy Booth

Teddy joined HouseFresh in 2019 as a senior content producer, producing written content as well as product photography and YouTube videos. He supports Danny as a product tester and is our lead how-to writer.

Join our newsletter

We’ll send you a nice email every once in a while. No spam.

Support HouseFresh

Buy us a coffee
Your support makes it possible for us to keep doing what we do.
We are ad-free and buy all the devices we test with our own money. When you buy through links on our site, we may earn an affiliate commission, which we use to fund new product tests. Learn more